1. Field of the Invention
The present invention relates to a control system for a multi-cylinder internal combustion engine, and more specifically, to a system for controlling the number of working (active) cylinders in a multi-cylinder engine and its operation.
2. Description of Prior Art
For improvement of fuel economy in multi-cylinder internal combustion engines for vehicles such as automobile, there has been proposed a strategy for controlling the number of working cylinders in an engine, depending upon a load applied thereon. A multi-cylinder engine has higher energy efficiency (the ratio of outputted mechanical energy to inputted fuel energy) when a load per cylinder is high. Thus, in this strategy, some of cylinders, e.g. a half number of cylinders, in an engine are selectively rendered inactive and a load per working cylinder is increased when the total load on the engine is low. This operation of a multi-cylinder engine while reducing the number of active cylinders is often called “cylinder reducing operation” or “partial operation”. Several examples of devices and methods executing cylinder reducing operation control for a multi-cylinder engine are seen in Japanese Patent Laid-Open Publications 05-272367, 7-180575, 11-182275, 11-350995, 5-332172, 6-193478, 10-103097, etc.
In cylinder reducing operation control as described in those publications, a total load on an engine is monitored through engine revolution speed and an accelerator or throttle opening or intake air pressure. Based upon a map of these parameters, the operation mode of an engine is switched between full cylinder operation, where all cylinders are active, and reduced cylinder operation, where only a half of cylinders are active and the rest of cylinders are inactive. Upon the switching of the operation mode, i.e. the changing of the number of working cylinders, a positive or negative surge of torque or power outputted from an engine occurs due to the variation of the mass of motion of an engine before and after the switching of the operation modes and the retardation of response of intake air flow amount variation. Such an output torque or power surge would cause a mechanical shock or impact on a driving system around an engine and a vehicle body, deteriorating the stability of the driving system and the drivability and driving stability of a vehicle. Thus, several strategies for avoiding a torque or output power surge upon the switching of the operation mode have been proposed also.
For instance, JP 5-332172, 6-193478 and 10-103097 disclose that ignition timing and throttle opening are varied for suppressing torque variation upon the switching of the operation mode. JP 5-272367 and 7-180575 each show that a map of engine revolution speed and an intake air pressure for determining an operation mode to be executed is modified depending upon a gear ratio of a transmission. JP 11-182275 and 11-350995 disclose a hybrid engine and dynamotor system where toque difference outputted from the engine before and after the changing of the number of active cylinders is cancelled through the operation of dynamotors.
Since the cylinder reducing operation control is employed mainly for increasing fuel efficiency of an engine, the engine should be operated in the reduced cylinder mode as long as torque or power requested of the engine, e.g. for driving a vehicle, can be generated by a reduced number of working cylinders. Ideally, the operation mode should be switched when the output power of an engine is at the maximum level available in the reduced cylinder mode. Then, each working cylinder will be operated at high load even when a total engine load is low.
Actual output torque or power from an engine, however, depends upon not only an intake air amount or throttle opening but also an engine temperature, environmental conditions of the engine, such as atmospheric pressure. It is difficult or cumbersome to estimate engine output torque/power precisely from an intake air amount, etc. Thus, when the operation mode at a certain condition is determined based upon only parameters such as throttle opening used so far, the switching of the operation mode will not always be executed at an ideal condition for providing output power requested in operating the engine while saving fuel. For instance, the premature switching of the operation mode into the reduced cylinder mode (and delayed switching into the full cylinder mode) would cause unexpected shortage of torque/power. On the other hand, when the switching into the reduced cylinder is too late, fuel would be wasted.
Accordingly, for further improvement of output performance and fuel economy of an engine, it will be preferable to provide a control strategy for cylinder reducing operation, enabling the switching of the operation mode at a more appropriate timing than ever, irrespective of the variation of internal and external (environmental) conditions of an engine.